Burner for coal, oil or gas firing

ABSTRACT

An improved burner for the combustion of coal, oil or gas places a plurality of retractable and rotatable gas elements in close proximity to the outlet end of a tubular burner nozzle of the burner which are shielded from combustion air provided around the tubular burner nozzle by a flame stabilizing ring having a plurality of openings adapted to closely receive each of the plurality of retractable and rotatable gas elements. Eddies produced by the flame stabilizing ring create a low oxygen/fuel rich flame resulting in reduced NO x  formation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fuel burners and, more particularly, toan improved burner for reducing the formation of nitric oxides duringthe combustion of pulverized, liquid or gaseous fuels.

One source of atmospheric pollution is the nitrogen oxides (NO_(x))present in the stack emission of fossil fuel fired steam generatingunits. Nitric oxide (NO) is an invisible, relatively harmless gas.However, as it passes through the vapor generator and comes into contactwith oxygen, it reacts to form nitrogen dioxide (NO₂) or other oxides ofnitrogen collectively referred to as nitric oxides. Nitrogen dioxide isa yellow-brown gas which, in sufficient concentrations, is toxic toanimal and plant life. It is this gas which may create the visible hazeat the stack discharge of a vapor generator.

Nitric oxide is formed as a result of the reaction of nitrogen andoxygen and may be thermal nitric oxide and/or fuel nitric oxide. Theformer occurs from the reaction of the nitrogen and oxygen contained inthe air supplied for the combustion of a fossil fuel whereas the latterresults from the reaction of the nitrogen contained in the fuel withoxygen in the combustion air.

The rate at which thermal nitric oxide is formed is dependent upon anyor a combination of the following variables; (1) flame temperature, (2)residence time of the combustion gases in the high temperature zone and(3) excess oxygen supply. The rate of formation of nitric oxideincreases as flame temperature increases. However, the reaction takestime and a mixture of nitrogen and oxygen at a given temperature for avery short time may produce less nitric oxide than the same mixture at alower temperature, but for a longer period of time. In vapor generatorsof the type hereunder discussion wherein the combustion of fuel and airmay generate flame temperatures in the order of 3,700° F., thetime-temperature relationship governing the reaction is such that atflame temperatures below 2,900° F. no appreciable nitric oxide (NO) isproduced, whereas above 2,900° F. the rate of reaction increasesrapidly.

The rate at which fuel nitric oxide is formed is principally dependenton the oxygen supply in the ignition zone and no appreciable nitricoxide is produced under a reducing atmosphere; that is, a conditionwhere the level of oxygen in the ignition zone is below that requiredfor a complete burning of the fuel.

It is apparent from the foregoing discussion that the formation ofthermal nitric oxide can be reduced by reducing flame temperatures inany degree and will be minimized with a flame temperature at or below2,900° F. and that the formation of fuel nitric oxide will be inhibitedby reducing the rate of oxygen introduction to the flame, i.e., air/fuelmixing.

In the U.S., Federal and state regulations are forcing development offossil fueled combustion equipment capable of reduced NO_(x) production.Lower NO_(x) emission requirements apply to pulverized, liquid andgaseous fuels, such as coal, oil and natural gas. While the energyshortages of the early 1970's have contributed to efforts forconservation of oil and gas, utilities in many areas of the country areunable to convert their oil and gas fired vapor generators to coalfiring due either to limitations of the existing equipment or due to theincreased particulate emissions attendant with coal vapor steamgenerators. In other situations, the need sometimes arises for a burnerhaving hardware capable of firing all three fuels, though notnecessarily more than one of these three fuels at a time. Accordingly, aneed exists for equipment capable of achieving reduced NO_(x) emissionswhen firing coal, oil and, in particular, natural gas, and which can beretrofitted to existing steam generator units.

2. Description of the Prior Art

Reducing NO_(x) emissions from fossil-fueled vapor generator units cantake several approaches. One approach uses fuels lower in nitrogencontent, if such flexibility is available. This only addresses part ofthe problem, however, and fails to address NO_(x) production arising outof the combustion process itself. Further, Federal and/or State emissionregulations may take the lower fuel bound nitrogen levels into accountwhen setting the standards to be met, and thus set a target level lowerthan what had to be met with the original fuel(s).

A second approach focuses on cleaning up the NO_(x) emissions producedby the combustion process itself, taking the nitrogen in the fuel andthe efficiencies of the burning of the fuel as given factors in theoverall process. One example of this is disclosed in U.S. Pat. No.4,309,386 to Pirsh, assigned to The Babcock & Wilcox Company. Pirshdiscloses a filter house that employs a selective catalytic reductionprocess for removing NO_(x) emissions from a flue gas stream whilesimultaneously filtering out and collecting entrained particulate matterfrom the stream. An extended treatment of both of the above approachesis beyond the scope and focus of the present application.

The third approach focuses upon the formation of NO_(x) emissions duringthe combustion process itself, and is what was referred to earlier asthermal nitric oxide and/or fuel nitric oxide. The combustion processinvloves the introduction of a fossil fuel and air into the furnace ofthe steam generator. Developments have thus focused on the fuel/airintroduction equipment, alone, as well as in combination with thefurnace of the steam generator.

Krippene, et al (U.S. Pat. No. 3,788,796), also assigned to The Babcock& Wilcox Company, is drawn to an improved pulverized fuel burnerapparatus and method for inhibiting the formation of fuel nitric oxideand providing the lower peak flame temperatures required to minimize theformation of thermal nitric oxide. Krippene, et al's burner is known inthe art as a dual register burner (DRB) because it employs two dampersor registers for separately apportioning and controlling combustion airflow between inner and outer annular passageways. The inner and outerannular passageways are concentrically placed around a central, tubularpulverized fuel nozzle. The pulverized fuel nozzle conveys a mixture ofpulverized fuel and combustion/transport air to the furnace where it isignited and burned with the rest of the combustion air flow provided bythe aforementioned inner and outer annular passageways.

Peterson, et al (U.S. Pat. No. 3,904,349), also assigned to The Babcock& Wilcox Company, is drawn to an improved liquid or gaseous fuel burnerapparatus having a central passageway, a first and a second annularpassageway, and separate means for apportioning the flow of combustionair among these passageways to achieve complete combustion of the fuelwhile reducing the formation of nitric oxides. The liquid fuel suppliedto and atomized within the burner is sprayed into the circular burnerport of the furnace in a pattern substantially symmetrical with the axisof the port. A central fuel tube or nozzle conveys the liquid or gaseousfuel to an atomizing assembly including a sprayer plate located at theoutlet end of the fuel tube or nozzle. The central fuel tube or nozzleextends through and out of a guide tube which supports at its distal enda truncated cone air deflecting device, through which the sprayer plateextends, which deflects combustion air conveyed by the centralpassageway and a portion of the combustion air conveyed by the firstannular passageway. Initial burning of the fuel is conducted in areducing zone by adjusting the quantity of combustion air dischargedthrough the central passageway; air admitted through the first annularpassageway causes recirculation of air about the outer periphery of thereducing zone to create a flame stabilizing zone; and finally, theremaining air for complete combustion is discharged through the secondannular passageway so as to envelop the reducing and stabilizing zonesand eventually mix with the fuel to complete its combustion.

LaRue, et al (U.S. Pat. No. 4,380,202), also assigned to The Babcock &Wilcox Company, is drawn to a mixer for a dual register burner for thecombustion of pulverized fuel. Instead of the venturi section andconical end-shaped rod member utilized in the apparatus of Krippene, etal, supra, a deflector and a diffuser having a plug and a shroud memberare located within the tubular pulverized fuel nozzle. As a result, flowseparation or fuel roping which can occur in the pulverized fuel nozzleis eliminated with minimum pressure loss effect on the primaryair/pulverized fuel stream.

As indicated earlier, another development to reduce NO_(x) formation inthe combustion of fossil fuels focuses on the combination/placement ofthe fuel/air burning equipment with respect to the furnace itself, andis known as two-stage combustion or TSC. TSC involves establishing alower, air deficient burner zone and an upper/downstream "after-air" or"over-fire-air" zone in the furnace. The amount of air by which thelower burner zone is deficient is injected in the over-fire-air zonedownstream to complete the combustion process. In essence, the wholefurnace is used as the combustion zone. A more refined version of TSCdeveloped as a result of strict NO_(x) emission limits in Japan and isknown as In-Furnace NO_(x) Reduction or IFNR. A description of thisprocess is contained in a paper entitled "Advanced In-Furnace NO_(x)Reduction Systems to Control Emissions" by M. A. Acree and A. D. LaRue,presented to the American Power Conference in Chicago, Illinois on Apr.22-24, 1985.

Briefly, the IFNR approach, jointly developed by Babcock-Hitachi K. K.and Tokyo Electric Power Company, employs multiple combustion zones inthe furnace. The main and lowest zone, the burner zone, utilizes lowNO_(x) burners operated at less than theoretical air levels to reducethe total amount of NO_(x) produced. The gases and char from this mainburner zone pass upwards into a reburning zone, that operates at evenlower air levels. Due to the low air levels, the fuel decomposes andforms hydrocarbon radicals that chemically combine to reduce the NO_(x)directly and which, in turn, further reduce the NO_(x) present. Uponleaving the reburning zone, since the NO_(x) levels in the flue gas havebeen reduced, the balance of the combustion air needed is introduced viaoverfire air parts in the combustion zone.

In new steam generator construction that applies either TSC or IFNRtechnology, the furnace volume and height are chosen to accommodate theextended combustion requirements so that the combustion products arecompletely burned before the flue gas passes across the radiant and/orconvective heat transfer tube banks of the vapor generator.

In a retrofit application, however, the furnace volume and height areusually not variable, and the optimum furnace dimensions needed forproper application of TSC or IFNR may not be available. These problemswere discussed in a paper entitled "Operating Experiences of Coal FiredUtility Boilers Using Hitachi NO_(x) Reduction Burners", By T. Narita,F. Koda, T. Masai, S. Morita, and S. Azuhata, presented at the 1987Joint Symposium on Stationary Combustion NO_(x) Control, in New Orleans,La., on Mar. 23-26, 1987, sponsored by the U.S. Environmental ProtectionAgency and the Electric Power Research Institute. As indicated in boththe Acree, et al and Narita, et al papers discussed above, generation oflow NO_(x) levels minimizes the amount of NO_(x) to be destroyeddownstream. Improvement of the existing dual register burner (DRB) ledto the development of what is known in the art as the Hitachi-NR burner(HTNR) for pulverized coal and the Primary Gas-Dual Register Burner(PG-DRB) for liquid and gaseous fuels.

Morita, et al (U.S. Pat. No. 4,545,307) is drawn to the improved HTNRburner mentioned above. In the prior art DRB (such as Krippene, et al)the pulverized coal stream is supplied with only enough air to transportthe coal; consequently, the burner flame at the burner throat entranceto the furnace formed a good reducing atmosphere. The balance of thecombustion air, called secondary and tertiary air, came to the burnerthroat via the inner and outer annular passageways, respectively, andwas to mix downstream of the central, reducing atmosphere burner flame.Too early mixing, however, of the secondary/tertiary air and thereducing atmosphere burner flame made maintaining the latter difficult.

Morita, et al modified the DRB for coal firing by attaching a bluff bodyat the outlet of the pulverized coal pipe, shaped as a ring-form dishhaving a hole therethrough for passing the pulverized coal/air mixtureinto the furnace. A portion or apron of the bluff body protrudes intothe inside diameter of the pulverized coal pipe to enhance ignitabilityat the exit thereof, while the outside diameter of the bluff bodyextends outside of the pulverized coal pipe partially into the secondaryair (inner annular) passageway. In addition, an outward guide sleeve isprovided, between the secondary air (inner annular) passageway and thetertiary (outer annular) passageway to dispense the tertiary airoutwards beyond the central pulverized coal flame, later combiningdownstream to complete the combustion process. The bluff body creates aneddy flow in the pulverized coal/air stream supplied by the pulverizedcoal pipe which prevents it from diffusing in an outward manner towardsthe secondary air stream.

Other development work on improved burners for coal firing has occurred,and is presented in a paper entitled "Development Status of B&W's SecondGeneration Low NO_(x) Burner--the XCL Burner", by A. D. LaRue, M. A.Acree and C. C. Masser, presented at the 1987 Joint Symposium onStationary Combustion NO_(x) control, in New Orleans, La., on Mar.23-27, 1987, sponsored by the U.S. Environmental Protection Agency andthe Electric Power Research Institute. The XCL burner design disclosedtherein while using criteria from the HTNR and DRB burners discussedearlier, was developed for coal firing only.

The Primary Gas-Dual Register Burner (PG-DRB) for oil and gas firing, isa DRB modified to include a recirculated gas annulus which surrounds aprimary air zone that houses the oil atomizer, and is disclosed in theAcree, et al reference mentioned above. It should be noted that, in thiscontext, the term "recirculated gas" refers to flue gas, rather thanfuel gas. The source of the recirculated gas would be from a pointsomewhere downstream of the last heat transfer surface in the steamgenerator, for example at the economizer outlet. The recirculated gasshields the base of the oil flame to reduce oxygen availability in theflame core; mixing of recirculated gas with the rest of the combustionair results in all of the combustion air having a lower oxygen contentto further suppress NO_(x) production.

For gas firing, as will be seen by a review of FIG. 4 of the Acree, etal reference, supra, and by referring to FIG. 1 of the presentapplication which shows a slightly modified version of the PG-DRB inschematic form, the gas elements 1 of the PG-DRB are placed in thetertiary air passageways 2 which encircle, successively, the oilatomizer 3, the primary air zone 4, the primary gas zone 5, and thesecondary air zone 6. While each gas element 1 has at the outlet endthereof shields 7 which protect the gas outlet nozzle 8 on each gaselement 1, it is clearly seen that each gas outlet nozzle 8 iscontinuously swept by the combustion air flow passing out into thefurnace 9 through the tertiary air passageways 2. This arrangementprevents the establishment of any fuel rich/low air reducing zone in thevicinity of each gas outlet nozzle 8 that is crucial for low NO_(x)emissions. In addition, some applications will also prohibit the use ofthe PG-DRB scheme, and yet the need for reducing NO_(x) emissions on gasfiring will remain.

Accordingly, it has become desirable to develop an improved burnerapparatus capable of separately firing pulverized, liquid or gaseousfuels and which can achieve reduced NO_(x) emissions on each of thesefuels.

SUMMARY OF THE INVENTION

The present invention provides an improved burner for the combustion ofcoal, oil or gas, and which achieves reduced NO_(x) emissions whenfiring any of these fuels.

Accordingly, an improvement is made on fuel burners of the typedesclosed in U.S. Pat. Nos. 3,788,796 and 4,545,307 and which isparticularly suited for retrofit applications to existing vaporgenerator units or for use in new construction of vapor generator units.Part of the improved burner is disposed within a windbox to which aportion of the necessary combustion air is supplied, and which is formedbetween adjacently disposed burner and furnace walls of the vaporgenerating unit. The burner wall is formed with an access opening foradmitting that portion of the improved burner that resides in thewindbox. The furnace wall is formed with a burner port that accommodatesthe combining of the fuel and air into a combustible mixture and theignition thereof. The ignited combustible mixture is then exhausted intothe combustion chamber or furnace of the vapor generator and is used toheat banks of tubes which contain water or steam.

The improved burner can fire coal, oil or gas, and includes separateelements which convey these fuels to the outlet end of the burner andother elements which convey the air needed for combustion to the outletend of the burner. Starting at the central axis of the burner andworking outwards, the improved burner includes a retractable oilatomizer, which conveys a mixture of oil and atomizing media to anatomizer sprayer plate located at the outlet end of the atomizer. Theatomizing sprayer plate is located at the outlet end of the burner. Theatomizer media, which can be air or steam, is needed to break the oil upinto droplets small enough so that they can be ignited. Mechanical, orpressure atomizers are also known and can be employed in place ofatomizers which use air or steam. Surrounding the retractable oilatomizer is a tubular burner nozzle. The oil atomizer is centrallylocated within the tubular burner nozzle and is supported by membersattached to the inside of the tubular burner nozzle.

The central passageway area between the outside of the oil atomizer andthe inside of the tubular burner nozzle conveys a mixture of primary airand pulverized coal particles to the outlet end of the burner and intothe furnace where this mixture will be ignited. Surrounding the tubularburner nozzle is a gas zone sleeve which partially defines an annularenclosure between it and the outside of the tubular burner nozzle.Attached to the outlet end of the tubular burner nozzle is a flamestabilizer ring, which together with the tubular burner nozzle and thegas zone sleeve completes the sides of the annular enclosure.

The flame stabilizing ring has two portions. A first portioncircumferentially extends around the outlet end of the tubular burnernozzle and partly into the central passageway, leaving an openinghowever through which the mixture of primary air and pulverized coal maypass. A second portion of the flame stabilizing ring is L-shaped, and isattached to the first portion of the flame stabilizing ring. This secondL-shaped portion extends circumferentially around and outwardly from theoutlet end of the tubular burner nozzle.

The annular enclosure contains a plurality of retractable and rotatablegas elements, which are concentrically arranged around the outside ofthe tubular burner nozzle. These gas elements extend through the annularenclosure, and convey fuel gas to the outlet end of the burner. Thesecond L-shaped portion of the flame stabilizing ring has a plurality ofholes or openings which allow the outlet ends of these gas elements topass through in close proximity to the outlet end of the tubular burnernozzle. The flame stabilizing ring protects or shields these outlet endsof the gas elements from secondary air which is being introduced aroundthe circumference of this area to reduce NO_(x) emissions. The secondaryair is provided to the outlet end of the burner through an inner annularpassageway and an outer annular passageway. The inner annular passagewayis defined as the space between the gas zone sleeve and an inner zonesleeve which encircles it. The outer annular passageway is defined asthe space between the inner zone sleeve and a burner barrel whichencircles the inner zone sleeve. To further direct the air exiting fromthe inner and outer annular passageways, an air separation vane ispresent which is connected to the outlet end of the inner zone sleeve.The air separation vane extends radially outwardly towards the furnaceand circumferentially around the outlet end of the inner zone sleeve.This air separation vane influences the path of the secondary air as itexits from the burner into the furnace. By shielding the outlet ends ofthe gas elements with the flame stabilizing ring through which they areinserted, a low oxygen/fuel rich flame is produced resulting in reducedNO_(x) formation.

Accordingly, one aspect of the present invention is drawn to an improvedburner for the combustion of coal, oil and in particular, gas, whichproduces low NO_(x) levels.

Another aspect of the present invention is drawn to a very particularflame stabilizing ring for a burner having a plurality of gas elementsarranged around and in close proximity to a tubular burner nozzle, andwhich acts to shield the outlet ends of the gas elements from thequantity of air for combustion transported by adjacent annularpassageways.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the presentinvention and the advantages attained by its use, reference is made tothe accompanying drawings and descriptive matter in which a preferredembodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing a prior art PG-DRB arrangement;

FIG. 2 is a schematic sectional elevation view of a vapor generatorusing fuel burning apparatus embodying the present invention;

FIG. 3 is a sectional elevation view of the improved fuel burner of thepresent invention;

FIG. 4 is a close-up view of the right hand portion of FIG. 3;

FIG. 5 is an end view of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to the drawings generally, wherein like numerals designate thesame element throughout the several drawings, and to FIG. 2 inparticular, there is shown a vapor generator 10 including water cooledwalls 12 which define a furnace chamber or combustion space 14 to whichthe fuel and air mixture is supplied by an improved burner 16. Aftercombustion has been completed in the furnace chamber 14, the heatedgases flow upwardly around the nose portion 18, over the tubularsecondary superheater 20, and thence downwardly through the convectionpass 22 containing the tubular primary superheater 24 and the economizer26. The gases leaving the convection pass 22 flow though the tubes of anair heater 28 and are thereafter discharged through a stack 30. It willbe understood that the heated gases passing over the superheaters 20 and24 and the economizer 26 give up heat to the fluid flowing therethroughand that the gases passing through the air heater 28 give up additionalheat to the combustion air flowing over the tubes. A forced draft fan 32supplies combustion air to the vapor generator 10 and causes it to flowover the air heater tubes and around a plurality of baffles 34 andthence through a duct 36 for apportionment between branch ducts 38 and40 respectively.

The air passing through duct 38 is delivered into a windbox 42 andrepresents, when firing coal, a major portion of the air necessary forcombustion of the fuel being discharged from a tubular burner nozzle 44associated with burner 16. The windbox air is proportioned between aninner annular passageway 90 and an outer annular passageway 94 fordicharge through a burner port 50 and into the furnace 14. For oilfiring, the burner 16 is provided with a retractable oil atomizer 46;similarly, for gas firing, the burner 16 is provided with a plurality ofretractable and rotatable gas elements 48. When firing oil or gas,substantially all of the air necessary for combustion flows through thewindbox 42; no substantial amount of air, other than nozzle sweep air,discussed infra, passes through the tubular burner nozzle 44. Theparticular details describing the oil atomizer 46 and the retractableand rotatable gas elements 48 will be presented infra.

The air passing through duct 40 is the remaining portion of airnecessary for combustion and is delivered into a primary air fan 52wherein it is further pressurized and thereafter conveyed through a duct54 into an air-swept type pulverizer apparatus 56.

Pulverized fuel to be burned in the vapor generator 10 is delivered inraw form via pipe 58 from the raw pulverizer fuel storage bunker 60 to afeeder 62 in response to the load demand on the vapor generator 10 in amanner well known in the art. The pulverizer 56 grinds the raw fuel tothe desired particle size. The pressurized air from primary air fan 52sweeps through the pulverizer 56 carrying therewith the ground fuelparticules fro flow through a pipe 64 and thence to the burner nozzle 44for discharge through the port 50 into the furnace 14.

A damper 66 is associated with the forced draft fan 32 to regulate thetotal quantity of air being admitted to the vapor generating unit 10, inresponse to the load demand. A damper 68 is associated with the primaryair fan 52 to regulate the quantity of air being introduced through theburner nozzle 44.

It will be appreciated that for the sake of clarity while the drawingsdepict one improved burner associated with one pulverizer wherein, inactual practice there may be more than one burner associated with apulverizer, and there may be more than one pulverizer associated withthe vapor generating unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 3, 4 and 5, there is shown the improved fuelburner 16 arranged to fire through the burner port 50. The burner port50 generally takes the shape of a frusto-conical throat which divergestowards the furnace side of the water cooled walls 12. In the burnerport 50 the tubes 70 which comprise the water cooled walls 12, are bentout of the plane of the wall 12 to form the burner port 50. An outerburner wall 72 having an access opening 74 is spaced from the watercooled walls 12. The space between the outer burner wall 72 and thefurnace walls 12 form the windbox 42.

The burner 16 includes the tubular burner nozzle 44 having an inlet endand an outlet end 44A and 44B respectively. The tubular burner nozzle 44defines a central passageway 45 and extends through an access openingcover plate 76, across the windbox 42 to a point adjacent the burnerport 50. The central passageway 45 defined by the tubular burner nozzle44, conveys a mixture of primary air and pulverized coal particles PA/PCto the outlet end of the burner 16. An elbow member 78 is flow connectedto the tubular burner nozzle inlet end 44A and at the other end to thepipe 64. Elbow member 78 includes a splash plate (end plate) 80 on itsoutside radius.

For oil firing, the burner 16 is provided with the retractable oilatomizer 46, having an inlet end 46A and an outlet end 46B, and which iscentrally disposed and supported within the central passageway 45. Theretractable oil atomizer 46 conveys a mixture of oil and atomizing mediato an atomizer sprayer plate 82 located at the outlet end 46B of theatomizer 46. The atomizer spray plate 82 disperses the atomized oil atthe outlet of the burner 16 in the vicinity of the burner port 50, andthence into the combustion chamber 14. The atomizing media, which can beair or steam, is needed to break the oil up into droplets which aresmall enough that they can be ignited, a function also performed by thedesign of the atomizer sprayer plate 82.

A gas zone sleeve 84 having an inlet end 84A and an outlet end 84B isconcentrically arranged around the tubular burner nozzle 44 andpartially defines an annular enclosure 86 therebetween. An inner zonesleeve 88 having an inlet end 88A and an oulet end 88B is concentricallyarranged around the gas zone sleeve 84 and defines an inner annularpassageway 90 therebetween. The inner annular passageway 90 conveys afirst portion of secondary air needed for combustion to the outlet endof the burner 16. A burner barrel 92 having an inlet end 92A and anoutlet end 92B is concentrically arranged around the inner zone sleeve88 and defines an outer annular passageway 94 therebetween. The outerannular passageway 94 conveys a second portion of the secondary airneeded for combustion to the outlet end of the burner 16. To accommodatedifferential expansion between the burner 16 and the furnace space orcombustion chamber 14, a seal 96 is provided at the attachment of theburner barrel 92 to the tubes 70.

It will be understood that in the present discussion, the term "primaryair" will refer to that portion of the combustion air which wouldaccompany or transport the pulverized coal particles during coal firing.The term "secondary air" refers to that portion of the combustion airwhich is provided through the inner and outer annular passageways 90, 94during firing of either coal, oil, or gas. This nomenclature is employedmerely for convenience; it is well known to those skilled in the artthat when firing oil or gas, substantially all of the air neccessary forcombustion flows through the windbox 42. When oil or gas is fired, nosubstantial amount of air, other than nozzle sweep air which will bedescribed below, passes through the tubular burner nozzle 44. Thus, inthe present discussion there can be secondary air without anyaccompanying primary air.

A sliding air sleeve 98, is attached to the inlet end 92A of the burnerbarrel for varying the cross-sectional flow area of a bell-mouthedannular opening 100 in the burner barrel 92 to regulate the amount ofsecondary air supplied to the inner and outer annular passageways 90,94. An annular burner barrel cover plate 102, having a lip 104, isattached to the inlet end 92A of the burner barrel and also to a tubularsleeve 106 which is attached to the access cover plate 76,

The sliding air sleeve 98 can be positioned to achieve a desiredcross-sectional air flow area of the bell-mouthed annular opening 100 bymeans of control rods (not shown) which would extend through the accessopening cover plate 76, allowing adjustments while the vapor generator10 is in operation. Such adjustments could be made either manually or bymeans of appropriately selected actuating devices. To facilitateappropriate selection of the position of the sliding air sleeve 98, agrid of pitot tubes 108 is located within the burner barrel 92downstream of the bell-mouthed annular opening 100 and upstream of theinner and outer annular passageways 90, 94, for measuring the totalsecondary air admitted to the burner 16.

A plurality of inner vanes 110 are concentrically arranged around thetubular burner nozzle 44 in the inner annular passageway 90, and imparta swirling action to the first portion of secondary air passing throughthe inner annular passageway 90. Similarly, a plurality of outer vanes112, 114 are concentrically arranged around the tubular burner nozzle 44in the outer annular passageway 94 to impart a swirling action to thesecond portion of the secondary air passing through the outer annularpassageway 94. Outer vane 114 may be fixed in place, if desired.Preferentially, the secondary air traveling through the outer annularpassageway 94 is swirled by two stages of outer vanes 112, 114.Providing two stages of outer vanes 112, 114 improves the efficiency ofswirl generation imparted to the secondary air passing through the outerannular passageway 94. If desired, adjustment of the position of theinner and outer vanes 110, 112 can be accomplished in the mannerdescribed in U.S. Pat. No. 4,380,202 to LaRue. An air separation vane116, is connected to and extends outwardly from and circumferentiallyaround the outlet end 88B of the inner zone sleeve 88. The airseparation vane 116 influences the path of the first portion ofsecondary air as it exits from the burner 16, in the manner disclosed inU.S. Pat. No. 4,545,307 to Morita, et al.

To provide for gas firing capability, a plurality of retractable androtatable gas elements 48 are concentrically arranged around the tubularburner nozzle 44 and extend through the annular enclosure 86. These gaselements 48 convey fuel gas to the outlet end of the burner 16, and havetheir outlet ends located in close proximity to the outlet end 44B ofthe tubular burner nozzle 44.

Referring also now to FIGS. 4 and 5, a flame stabilizing ring 118 isattached to the outlet end 44B of the tubular burner nozzle 44.Together, the tubular burner nozzle 44, the gas zone sleeve 84, and theflame stabilizing ring 118 define the annular enclosure 86 therebetween.The flame stabilizing ring 118 has a first portion 120 whichcircumferentially extends into the central passageway 45 of the tubularburner nozzle 44 end defines an opening 122 through which the mixture ofprimary air and pulverized coal particles would pass during coal firing.The first portion 120 can be a continuous surface or it can be providedwith a plurality of serrations or teeth 124 as is disclosed in U.S. Pat.No. 4,545,307 to Morita, et al. Attached to the first portion 120 of theflame stabilizing ring 118 is a second, L-shaped portion 126 whichextends circumferentially around and outwardly from the outlet end 44Bof the tubular burner nozzle 44. A plurality of holes or openings 128,equal in number to the number of retractable and rotatable gas elements48, are provided in the second, L-shaped portion 126. These openings 128are adapted to closely receive therethrough each of the plurality ofretractable gas elements 48. By providing this flame stabilizing ring118 having the plurality of openings 48, the retractable and rotatablegas elements 48 are shielded from the secondary air which passes throughthe inner and outer annular passageways 90, 94, resulting in reducedNO_(x) emissions when gas is fired in the burner 16. The reduced NO_(x)reduction is caused by eddies produced by the flame stabilizing ring 118which create a low oxygen/fuel rich flame. Particular sizing criteria ofthe overall dimensions of the flame stabilizing ring 118 would followgenerally the criteria set forth in U.S. Pat. No. 4,545,307 to Morita,et al for what is described therein as the "bluff body".

When coal is fired in the improved burner 16, the retractable androtatable gas elements 48, as well as the reatractable oil atomizer 46are retracted to a position behind the flame stabilizing ring 118 toprotect these components from overheating or fouling by deposits fromthe coal. For the case of gas or oil firing in the improved burner 16,the retractable oil atomizer 46 or the plurality of retractable androtatable gas elements 48 are inserted into the burner port 50 so thatthey extend beyond the plane of the first portion 120 of the flamestabilizing ring 118. In the firing of oil or gas, a very small quantityof secondary air would be admitted to the tubular burner nozzle 44through a nozzle air duct 130, which is shown in FIG. 3. This smallquantity of secondary air or nozzle sweep air is admitted to the tubularburner nozzle 44 to prevent backflow of combustion byproducts into thetubular burner nozzle 44.

For the case of gas firing in the improved burner 16, additionalelements are necessary to provide the fuel gas from a source (not shown)to the plurality of retractable and rotatable gas elements 48. Aplurality of gas element support pipes 132 are sleeved around each ofthe plurality of gas elements 48. These gas element support pipes 132position the gas elements 48 with respect to the flame stabilizing ring118. Positioning of each of the gas elements 48 may involve moving thegas elements 48 towards or away from the furnace combustion chamber 14,rotation of the gas elements 48 about the longitudinal axis thereof, orany combination thereof. Attached to each of the plurality of gaselements 48 is a flexible gas hose 134 which allows for rotation,insertion and retraction of each of the gas elements 48. Finally, a gasmanifold 136 provides a common source of fuel gas to which each of theplurality of gas hoses 134 is attached. Preferentially, the gas manifold136 is located outside of the windbox 42 for ease of access and/orrepair. This arrangement further facilitates servicing of the gaselements 48, since they can be removed from the burners 16 forinspection or cleaning by this arrangement while the vapor generator 10is in service.

An ignitor assembly 138 of known construction would be provided toignite the combustion mixture of fuel and air provided at the outlet ofthe burner 16. As shown in FIG. 3, the ignitor assembly 138 would extendthrough the access opening cover plate 76 through the windbox 42 andterminate in the vicinity of the burner port 15. An actuator 140 can beapplied to the ignitor 138 for automatic positioning thereof. Similarly,an actuator 142, attached to the inlet end of the oil atomizer 46A andto the splash plate 80, can be used to position the oil atomizer 46. Inthe same manner, atutomatic actuator means (not shown) can be used toposition the plurality of gas elements 48 with respect to the flamestabilizing ring 118. Whether the positioning of the gas elements 48 isby manual or automatic means however, the critical feature to beobserved is that the outlet ends 48B of each of the gas elements 48should be placed during gas firing such that they receive the fullbenefit of the shielding effect from the secondary air that is providedby the flame stabilizing ring 118. Finally, the improved burner 16 canemploy a frusto-conical difusser 144 disposed within the inlet end 44Aof the tubular burner nozzle 44 and which has an opening 146 throughwhich the retractable oil atomizer 46 can pass. The frusto-conicaldiffuser 144 would disperse the majority of the pulverized coalparticles entrained in the primary air to a location near the insidesurface 148 of the tubular burner nozzle 44, leaving the central portionof the central passageway 45 relatively free of pulverized coalparticles. Particular design features of the frusto-conical diffuserwould be applied as disclosed in U.S. Pat. No. 4,380,202 to LaRue.

The flame stabilizing ring 118 has a significant effect on the mixtureof primary air and pulverized coal particles during coal firing. Theflame stabilizing ring 118 causes recirculation of this streamtherewithin in a manner which promotes ignition of the coal particlesand improved flame stability. The arrangement of the gas zone sleeve 84with its junction at the flame stabilizing ring 118 in combination withthe tubular burner nozzle 44 prevents the flow of any secondary airthrough the annular enclosure 86. As a consequence, the flame formed bythe ignited pulverized coal fuel jet in the flame stabilizing ring 118together with the regulated introduction of secondary air through theinner and outer annular passageways 90, 94 produces very low NO_(x)emissions when firing coal, as disclosed in U.S. Pat. No. 4,545,307 toMorita, et al.

For the case of oil firing, the ignited fine mist of fuel oil dropletsproduced by the atomizer spray plate 82 located in the center of theflame stabilizing ring 118 also achieves reduced NO_(x) formationlevels. Flow profiles produced by the secondary air traveling throughthe inner and outer annular passageways 90, 94 are affected by the flamestabilizing ring 118 and air separation vane 116 to reduce oxygenavailability to the oil flame in a zone immediately downstream of theburner port 50, which acts to inhibit formation of NO and NO₂. Completemixing of the secondary air with the partially burned oil fuel occursfurther downstream in a series of reactions at lower temperatures andlower oxygen partial pressures such that NO_(x) formation is avoided asthe char reactions are completed. Combustion tests indicate an abilityto reduce NO_(x) at typical excess air levels from uncontrolled levelsof 200 ppm to levels in the range of 120 ppm, without two stagecombustion or gas recirculation when firing oil. By uncontrolled, theburner 16 was operated without the flame stabilizing ring 118 or the airseparation vane 116.

For the case of gas firing, the arrangement of retractable and rotatablegas elements 48 positioned to be shielded by the flame stabilizing ring118 results in an extremely stable flame. Combustion tests havedemonstrated stable combustion from levels of 100% burner input tolevels less than 10% maximum input without excessive flame inducedvibration or rumble. With respect to NO_(x) emissions when firing gas,no uncontrolled tests of the burner 16 without the flame stabilizing 118or the air separation vane 116 were performed. Combustion tests wereperformed with the elements, however, and NO_(x) levels were see in therange of 130-150 ppm. Based upon experience with previous designburners, and upon a scaling factor to compare the uncontrolled oilfiring performance to a hypothetical uncontrolled gas firingperformance, it is estimated that the burner 16 would produce NO_(x)levels of approximately 190 ppm in an uncontrolled (no flame stabilizingring 118 or air separation vane 116) setting under similar conditions,showing the burner 16 NO_(x) reduction capability when firing gas.

While in accordance with provisions of the statutes there is illustratedand described herein a specific embodiment of the invention, thoseskilled in the art will understand that changes may be made in the formof the invention covered by the following claims, and that certainfeatures of the invention may sometimes be used to advantage without acorresponding use of the other features. For example, the improvedburner can be arranged to fire only one, or two, of the fuels bydeleting the unnecessary elements. In all cases the burner nozzle isretained fully or in part, but would not be used to transport fuel ofcoal is not fired.

I claim:
 1. A burner for the combustion if coal, oil or gas,comprising:a tubular burner nozzle having an inlet end and an outlet endand which defines a central passageway for conveying primary aircontaining pulverized coal particles therethrough to an outlet end ofthe burner; a gas zone sleeve having an inlet end and an outlet end,concentrically arranged around the tubular burner nozzle and partiallydefining an annular enclosure therebetween; an inner zone sleeve havingan inlet end and an outlet end, concentrically arranged around the gaszone sleeve and defining an inner annular passageway therebetween, forconveying a first portion of secondary air needed for combustion to theoutlet end of the burner; a burner barrel having an inlet end and anoutlet end, concentrically arranged around the inner zone sleeve anddefining an outer annular passageway therebetween, for conveying asecond portion of secondary air needed for combustion to the outlet endof the burner; an air separation vane, connected to an extendingoutwardly and circumferentially around the outlet end of the inner zonesleeve, for influencing the path of the first portion of secondary airas it exits from the burner; a retractable oil atomizer having an inletend and an outlet end, centrally disposed and supported within thecentral passageway, for conveying a mixture of oil and atomizing mediato an atomizer sprayer plate located at the outlet end of the atomizerat the outlet end of the burner; a plurality of retractable androtatable gas elements, concentrically arranged around the tubularburner nozzle and extending through the annular enclosure, for conveyinggas to the outlet end of the burner, having outlet ends located in closeproximity to the outlet end of the tubular burner nozzle, and beingshielded from the secondary air when fully inserted into the burner by;a flame stabilizing ring, attached to the outlet end of the tubularburner nozzle and which together with the tubular burner nozzle and thegas zone sleeve defines the annular enclosure therebetween, having afirst portion circumferentially extending into the central passageway todefine an opening therein, a second L-shaped portion attached to thefirst portion of the flame stabilizing ring extending circumferentiallyaround and outwardly from the outlet end of the tubular burner nozzle,and having a plurality of openings adapted to closely receivetherethrough each of the plurality of retractable gas elements.
 2. Anapparatus according to claim 1, further including:first and second meansfor retracting, respectively, the oil atomizer and its attached sprayerplate, and the plurality of gas elements, to a position behind the flamestabilizing ring when coal is fired to protect the oil atomizer, thesprayer plate and the gas elements from overheating or fouling.
 3. Anapparatus according to claim 2, further including:a sliding air sleeveattached to the inlet end of the burner barrel, for varying thecross-sectional air flow area of a bell-mouthed annular opening in theburner barrel to regulate the amount of secondary air supplied to theinner and outer annular passageways.
 4. An apparatus according to claim3, further including:a nozzle air duct, attached to the tubular burnernozzle, for supplying a quantity of secondary air to the tubular burnernozzle sufficient to prevent backflow of combustion byproducts thereintowhen the burner is firing oil or gas.
 5. An apparatus according to claim4, further including:a frustoconical diffuser, disposed within the inletend of the tubular burner nozzle and having an opening through which theretractable oil atomizer passes, to disperse the majority of thepulverized coal particles in the primary air near an inside surface ofthe tubular burner nozzle, leaving the central portion of the centralpassageway relatively free of pulverized coal particles.
 6. Apparatusaccording to claim 1, further including:a plurality of gas elementsupport pipes, sleeved around each of the plurality of gas elements, forpositioning the gas elements with respect to the flame stabilizing ring;a plurality of flexible gas hoses, attached to each of the plurality ofgas element support pipes, for conveying fuel gas to, and allowingrotation, insertion and restriction of, each gas element support pipe;and a gas manifold, to which each of the plurality of gas hoses isattached, for supplying fuel gas to each of the gas hoses.
 7. Apparatusaccording to claim 6, further including:a retractable lighter, arrangedfor insertion into the inner annular passageway to a point at the outletend of the burner so as to ignite a fuel and air mixture.
 8. Apparatusaccording to claim 1, further including:a plurality of inner vanes,concentrically arranged around the tubular burner nozzle in the innerannular passageway, to impart a swirling acion to the first portion ofsecondary air as it leaves the inner annular passageway; a plurality ofouter vanes, concentrically arranged around the tubular burner nozzle inthe outer annular passageway, to impart a swirling action to the secondportion of the secondary air as it leaves the outer annular passageway;and a grid of pitot tubes, located within the burner barrel downstreamof the bell-mouthed annular opening thereinto and upstream of the innerand outer annular passageways, for measuring the total secondary airadmitted to the burner.
 9. A flame stabilizing ring for a burner havinga plurality of gas elements concentrically arranged around and in closeproximity to a tubular burner nozzle having a central passageway andoutlet end, and at least one annular passageway concentrically arrangedaround the plurality of gas elements for transporting a quantity of airto an outlet end of the burner for combustion, comprising:a firstportion, attached to the outlet end of the tubular burner nozzle andcircumferentially extending into the central passageway to define anopening therein; and a second, L-shaped portion attached to the firstportion of the flame stabilizing ring extending circumferentially aroundand outwardly from the outlet end of the tubular burner nozzle, having aplurality of openings adapted to closely receive therethrough each ofthe plurality of gas elements, such that outlet ends of each of theplurality of gas elements is shielded from the quantity of air forcombustion transported by the at least one annular passageway.
 10. Aburner for combustion of oil or gas, comprising:a tubular burner nozzlehaving an inlet end and an outlet end, an outlet being located at theoutlet end of the burner; a gas zone sleeve having an inlet end and anoutlet end, concentrically arranged around the tubular burner nozzle andpartially defining an annular enclosure therebetween; an inner zonesleeve having an inlet end and an outlet end concentrically arrangedaround the gas zone sleeve and defining an inner annular passagewaytherebetween, for conveying a first portion of secondary air needed forcombustion to the outlet end of the burner; a burner barrel having aninlet end and an outlet end, concentrically arranged around the innerzone sleeve and defining an outer annular passageway therebetween, forconveying a second portion of secondary air needed for combustion to theoutlet end of the burner; an air separation vane, connected to andextending outwardly and circumferentially around the outlet end of theinner zone sleeve, for influencing the path of the first portion ofsecondary air as it exits from the burner; a retractable oil atomizerhaving an inlet end and an outlet end, centrally disposed and supportedwithin the central passageway, for conveying a mixture of oil andatomizing media to an atomizer sprayer plate located at the outlet endof the atomizer at the outlet end of the burner; a plurality ofretractable and rotatable gas elements, concentrically arranged aroundthe tubular burner nozzle and extending through the annular enclosure,for conveying gas to the outlet end of the burner, having outlet endslocated in close proximity to the outlet end of the tubular burnernozzle, and being shielded from the secondary air when fully insertedinto the burner by: a flame stabilizing ring, attached to the outlet endof the tubular burner nozzle and which together with the tubular burnernozzle and the gas zone sleeve defines the annular enclosuretherebetween, having a first portion circumferentially extending intothe central passageway to define an opening therein, a second L-shapedportion attached to the first portion of the flame stabilizing ringextending circumferentially around and outwardly from the outlet end ofthe tubular burner nozzle, and having a plurality of openings adapted toclosely receive therethrough each of the plurality of retractable gaselements.
 11. A burner for combustion of coal or gas, comprising:atubular burner nozzle having an inlet end and an outlet end and whichdefines a central passageway for conveying primary air containingpulverized coal particles therethrough to an outlet end of the burner; agas zone sleeve having an inlet end and an outlet end, concentricallyarranged around the tubular burner nozzle and partially defining anannular enclosure therebetween; an inner zone sleeve having an inlet endand an outlet end concentrically arranged around the gas zone sleeve anddefining an inner annular passageway therebetween, for conveying a firstportion of secondary air needed for combustion to the outlet end of theburner; a burner barrel having an inlet end and an outlet end,concentrically arranged around the inner zone sleeve and defining anouter annular passageway therebetween, for conveying a second portion ofsecondary air needed for combustion to the outlet end of the burner; anair separation vane, connected to and extending outwardly andcircumferentially around the outlet end of the inner zone sleeve, forinfluencing the path of the first portion of secondary air as it exitsfrom the burner; a plurality of retractable and rotatable gas elements,concentrically arranged around the tubular burner nozzle and extendingthrough the annular enclosure, for conveying gas to the outlet end ofthe burner, having outlet ends located in close proximity to the outletend of the tubular burner nozzle, and being shielded from the secondaryair when fully inserted into the burner by: a flame stabilizing ring,attached to the outlet end of the tubular burner nozzle and whichtogether with the tubular burner nozzle and the gas zone sleeve definesthe annular enclosure therebetween, having a first portioncircumferentially extending into the central passageway to define anopening therein, a second L-shaped portion attached to the first portionof the flame stabilizing ring extending circumferentially around andoutwardly from the outlet end of the tubular burner nozzle, and having aplurality of openings adapted to closely receive therethrough each ofthe plurality of retractable gas elements.
 12. A burner for combustionof gas, comprising:a tubular burner nozzle having an inlet end and anoutlet end, the outlet end being located at an outlet end of the burner;a gas zone sleeve having an inlet end and an outlet end, concentricallyarranged around the tubular burner nozzle and partially defining anannular enclosure therebetween; an inner zone sleeve having an inlet endand an outlet end concentrically arranged around the gas zone sleeve anddefining an inner annular passageway therebetween, for conveying a firstportion of secondary air needed for combustion to the outlet end of theburner; a burner barrel having an inlet end and an outlet end,concentrically arranged around the inner zone sleeve and defining anouter annular passageway therebetween, for conveying a second portion ofsecondary air needed for combustion to the outlet end of the burner; anair separation vane, connected to and extending outwardly andcircumferentially around the outlet end of the inner zone sleeve, forinfluencing the path of the first portion of secondary air as it exitsfrom the burner; a plurality of retractable and rotatable gas elements,concentrically arranged around the tubular burner nozzle and extendingthrough the annular enclosure, for conveying gas to the outlet end ofthe burner, having outlet ends located in close proximity to the outletend of the tubular burner nozzle, and being shielded from the secondaryair when fully inserted into the burner by: a flame stabilizing ring,attached to the outlet end of the tubular burner nozzle and whichtogether with the tubular burner nozzle and the gas zone sleeve definesthe annular enclosure therebetween, having a first portioncircumferentially extending into the central passageway to define anopening therein, a second L-shaped portion attached to the first portionof the flame stabilizing ring extending circumferentially around andoutwardly from the outlet end of the tubular burner nozzle, and having aplurality of openings adapted to closely receive therethrough each ofthe plurality of retractable gas elements.